In semi-autogenous grinding mills and also in fully autogenous grinding mills, part (in the case of semi-autogenous mills) or all (in the case of fully autogenous mills) of the grinding is performed by "media size rock" having a rough diameter of approximately 4" to 12" and which is part of the ore being fed to the mill. When the input feed to the mill contains a sufficient quantity of the media size rock as just defined, then the input feed ore is considered for purposes of this specification to be "satisfactory" ore. Conversely, when there is not a sufficient amount of the media size rock in the input ore feed to the autogenous or to the semi-autogenous grinding mill, then the input ore is considered, for purposes of this specification, to be "unsatisfactory" ore.
In a very high percentage of mining operations, the ore delivered from the mine to the grinding mill contiguous to the mine varies unpredictably between what has been defined as "satisfactory" ore and what has been defined as "unsatisfactory" ore. It should be noted that the terms "satisfactory" and "unsatisfactory" ore refer to the geometry, size, and quantity of the pieces of ore in the media size range--that is, ore larger than 4" in size in the ore feed as it is fed to the grinding mill. The mineralogical characteristics of the ore can be and frequently are independent of the autogenous characteristics referred to as "satisfactory" ore and the "unsatisfactory" ore.
In the semi-autogenous grinding mill, metallic balls which typically are steel balls having a diameter in the range of 4" to 5" are used to supplement the grinding action of the "rock grinding media" defined by the larger size rocks in the input feed to the mill. Such a 5" steel ball would normally weigh approximately 18 pounds. The steel balls are much more dense than the rock grinding media and have much greater impact force than the rock grinding media.
A serious problem arises when a primary semi-autogenous grinding mill is being used for grinding mineral ore and either the mining procedure and/or the ore storage facilities upstream of the semi-autogenous grinding mill or, alternatively, the process downstream of the semi-autogenous grinding mill, cannot be adapted to handle the potentially wide variations in the ore thruput through the primary semi-autogenous grinding mill which is inherently necessary for satisfactory operation of the mill.
It is not practical to operate primary semi-autogenous grinding mills with a constant or uniform ore feed rate to the mill since with a constant feed rate to a primary semi-autogenous grinding mill when "satisfactory" ore (i.e., ore having good autogenous grinding characteristics) is the input feed to the grinding mill, the volumeteric load in the semi-autogenous mill can drop to an undesirably low level due to the greater autogenous grinding efficiency of the "satisfactory" ore. This drop in volumeteric load in the primary semi-autogenous grinding mill causes the metallic grinding balls used as supplemental grinding media in the mill to become exposed and also to become a larger proportion of the total charge in the mill. Exposure of the metallic grinding balls used as a supplemental grinding media causes greatly increased breakage of the balls due to ball-to-ball action. Also, as the metallic grinding balls become a greater percentage of the charge and become more exposed, they will impact to a greater degree on the mill lining, causing breakage of the mill lining. As the size of the "toe" in the charge is reduced, this increases the amount of breakage.
The problem just described in connection with the exposure of the metallic grinding balls in a semi-autogenous grinding mill is a particular problem in such mills because of the substantially higher peripheral velocity of a semi-autogenous grinding mill than conventional ball or rod mills. Autogenous and semi-autogenous grinding mills are typically 20' to 36' in diameter, as compared to a typical 15' to 18' diameter for conventional ball or rod mills. Because of their larger diameter, semi-autogenous mills have a substantially greater peripheral velocity than conventional ball or rod mills, as just mentioned. This higher peripheral velocity results in higher impact force of the balls on each other and on the liners when there are low "pulp" (i.e., slurry consisting of ore and water) levels in the semi-autogenous mill, with resulting damage to the balls and liners.
In view of the foregoing, it is much more practical to operate a primary semi-autogenous grinding mill at a predetermined constant power input and variable ore feed input to the mill, rather than at constant feed, variable power draw on the mill since with the constant input power, variable ore feed operation, the level of the pulp (i.e., the slurry consisting of the ore and water) in the semi-autogenous grinding mill can be constantly maintained at a predetermined optimum volumetric level in the mill so as to minimize breakage of the grinding balls and the mill liners.
It is known in the art of grinding mineral ores that semi-autogenous grinding mills have certain advantages as compared to fully autogenous grinding mills. These advantages may be briefly summarized as follows:
(1) The semi-autogenous grinding mill requires less input power per ton of the ore being ground;
(2) In most instances, the semi-autogenous grinding mill produces less fines or very fine grinding product than does the fully autogenous grinding mill. This could be an advantage in many processes for which the grinding mill output product is feed.
(3) Because of the use of metallic grinding balls, such as steel balls in the semi-autogenous grinding mill, there is more available grinding power in a semi-autogenous mill of a given size than in a fully autogenous grinding mill of the same size and volumeteric loading. This is due to the fact that the grinding action in a fully autogenous grinding mill is completely dependent upon the rock grinding media whereas in a semi-autogenous grinding mill steel balls having a much higher density or specific gravity than the rock grinding media are used to supplement the grinding action of the rock grinding media thereby providing greater grinding energy per unit volume in the semi-autogenous grinding mill than in the fully autogenous grinding mill.
(4) A further advantage of the semi-autogenous grinding mill is that there are more mineral ores suitable for semi-autogenous grinding than are available for fully autogenous grinding.
Because of the aforementioned advantages of a semi-autogenous grinding mill, it is often desirable to employ a semi-autogenous grinding mill for the grinding operation and yet the semi-autogenous grinding mill as a practical matter is only adapted for use when the mill is being supplied from a supply source which is capable of supplying the input feed ore to the grinding mill at a variable feed rate. However, certain types of mining operations are not adapted to provide a variable feed rate to the grinding mill. For example, an underground mine which depends for transport of the output of the mine upon hoists, conveyors, etc., cannot easily be accommodated to the variable input requirements of a semi-autogenous grinding mill but is better suited for delivery of a constant input feed rate to the grinding mill. On the other hand, an open pit mine where the ore can be hauled by rail or truck is better adapted to supply a variable ore feed rate input to the autogenous or semi-autogenous grinding mill.